I used my bathtub in a similar manner in the early 1980s. The bathroom had a window with wooden surrounds. I sealed the window with medium density fibre board [MDF] coated with black felt. I stapled the felt to the MDF, and put a handle on the MDF so that I could easily pull the "blackout" from the window. The bathroom door was easily sealed with weather foam. I was even able to put a darkroom red light in the light fitting of the bathroom. Only trouble was that the whole thing had to be dismantled if I wanted to have a shower - which, being Australian, I do daily!!